This invention relates to brushless D.C. motors used for the propulsion of vehicles. More particularly, this invention relates to a brushless D.C. vehicle propulsion motor with a more efficient design using movable magnet poles.
Brushless D.C. vehicle propulsion motors are known and have been used for the propulsion of many different types of vehicles, such as bicycles, motorcycles, autos, and small trucks. A typical motor design has a rotor and a stator. The rotor is fixedly attached to the vehicle wheel for rotation therewith; the stator is attached to a vehicle stationary member, such as the fork of a bicycle or motorcycle frame. A specific type of brushless D.C. motor is a disk motor. In a disk motor, both the rotor and the stator typically comprise disks having circular geometry, with the rotor disk being rotationally arranged between two flanking stator disks. The rotor disk usually carries a plurality of permanent magnets mounted along a circular path centered on the rotational axis of the rotor disk. In some disk motors the permanent magnets are mounted along only one circular path; in others, the permanent magnets are mounted along two or more concentric circular paths. The stator disks are fixedly mounted to the vehicle and each stator disk carries a plurality of electromagnets distributed in one or more matching circular paths centered on the axis of the stator disk with essentially the same radii as the circular paths of the permanent magnets on the rotor disk. The coils of the electromagnets are typically coupled to a multi-phase driving circuit, usually in a three-phase arrangement. Electrical power for the driving circuit is supplied by a D.C. power source, such as a lead-acid battery, and a power conversion circuit is used to convert the D.C. electrical power from the battery to multi-phase pulse or A.C. power signals for synchronously driving the electromagnets mounted on the stator disks to provide rotating magnetic fields which interact with the rotor permanent magnets to provide the driving forces for the rotor. Typically, the electromagnets are grouped in phases, with all electromagnets in the same phase group being driven in unison and electromagnets in different phase groups being driven with differently phased power signals. A manually operable control circuit allows the frequency or the duty cycle of the power signals produced by the driving circuit to be varied, which causes the rotor to be driven at different rotational speeds by the rotating magnetic fields produced by the electromagnets. Rotor position signals generated by individual position sensors (such as Hall effect sensors) mounted adjacent the rotor at different angular positions, or by back EMF sensor circuits connected to the coils, provide position information to govern the switching of the power signals to the next commutation state. A motor speed feedback signal is supplied to the control electronics. For a general discussion of brushless D.C. motor propulsion techniques, reference may be has to Application Note AVR: 443 entitled “Sensor-based control of three phase Brushless DC motor” published by Atmel Corporation of San Jose, Calif. Examples of known multi-phase A.C. vehicle propulsion motors are shown in U.S. Pat. Nos. 6,100,615; 6,276,475 and 6,617,746, and U.S. Patent Application Publication Number US 2002/0135220 A1, the disclosures of which are hereby incorporated by reference.
The basic disk motor configuration described thus far can be expanded to include several rotors and stators laterally spaced along the rotational axis of the disk motor. In such configurations, the driving circuit remains essentially the same, with multi-phase power signals being applied in parallel to the electromagnets mounted on the several stator plates.
In all known disk motor power control systems, the multi-phase pulse power signals are applied to all of the electromagnets in the stator disks, regardless of the actual vehicle speed or load demand on the disk motor. As a consequence, the energy demand on the battery power source is usually greater than that actually required by the disk motor in order to provide the propulsion force ideally required under a given set of vehicle speed or load conditions. This excessive use of battery power unduly limits the range of the associated vehicle and thus the performance of known brushless D.C. motor vehicle propulsion systems.
Commonly assigned U.S. patent application Ser. No. 12/589,916 filed Oct. 30, 2009 for “Power Control System For Vehicle Disk Motor” discloses a power control system and method for brushless D.C. vehicle disk motors which is devoid of the limitations noted above in known disk motor power control designs, and which is therefore capable of affording greater vehicle range on a given battery charge and providing greater vehicle range for a battery of given energy storage capacity. According to the power control techniques disclosed therein, the stator electromagnets are grouped into sets, with all of the electromagnets located along a given circular path being assigned to a particular set. In operation, the electromagnet coils are activated on a selective basis based on a the value of a vehicle condition parameter, such as vehicle speed or load on the disk motor, as well as demanded vehicle speed. Electric vehicle propulsion systems using this technique are capable of being operated in a much more efficient manner than disk motors in which the stator coils are operated continuously in parallel. Specifically, only those stator set coils which are necessary to provide the optimum propulsion force to the vehicle are activated. Thus, when maximum power is required (e.g., when a vehicle is starting from a standstill), the stator coils in all of the stator coil sets are activated. When maximum power is no longer required (e.g., the vehicle reaches a first set speed), the stator coils in less than all of the stator coil sets are activated. During this selective operating mode, at least some of the coils are deactivated, thereby drawing no electrical power from the energy source. This selective operating mode is conducted during the majority of the total operating time of the power control system and thereby extends the useful life of the electrical energy stored in a battery power source. Consequently, a smaller battery can be used in an electrically powered vehicle propulsion system to obtain the same range as a vehicle disk motor using conventional stator coil activation techniques. In addition, given a battery of a specific energy capacity, a disk motor operated in accordance with this technique can achieve a longer range than a disk motor operated according to conventional techniques.
Due to the relatively low magnetic permeability of air, disk motors are designed with a close spacing between the opposing faces of the permanent magnets mounted on the rotor and the electromagnets mounted on the stators. This provides maximum interaction between the magnetic fields generated by the permanent magnets and the electromagnets. When a disk motor is operated with power control signals using the selective coil set activation technique described above, during the majority of the total operating time of the power control system the permanent rotor magnets move past the deactivated electromagnet coils and induce magnetic fields in the electromagnet pole pieces. This has a retarding effect on the rotor motion, which is compounded by the close spacing between the opposing faces of the permanent magnets and the electromagnets. As a result, the efficiency of the disk motor is less than optimal.